1. Field of the Invention
This invention relates to an improved wire matrix print head. A wire matrix print head generally includes a needle guide assembly and a plurality of electromagnetic structures coupled to the guide assembly. Each electromagnetic structure is mounted on an electromagnetic support and comprises a magnetic circuit formed by two pole pieces connected by a yoke and by a movable armature which completes the magnetic circuit. It further includes at least a coil wound around one of the two pole pieces.
2. Description of the Prior Art
An example of a wire matrix print head is given in U.S. Pat. No. 4,051,941. Note that each of the electromagnetic structures acts on one of the printing needles. Furthermore each movable armature is provided with at least an arm which protrudes externally to the electromagnetic structure and operates on an actuating arm for the associated needle.
Such wire print heads must be very inexpensive and simple in construction. At the same time they must allow for precise adjustment, in particular of the air gap width in the electromagnetic structure with the armature at rest and on the distance of the needle's tip from the platen when needles are at rest. Italian Pat. No. 956,585 describes a wire matrix print head where such adjustments can be individually performed on each electromagnetic structure and related needle. However such print head is complex and expensive. In contrast the number of constructive elements and constructive complexity are reduced to a minimum, in the wire matrix print head disclosed in U.S. Pat. No. 4,051,941 at the expense of adjustability and calibration.
Further the constructive arrangement shown in U.S. Pat. No. 4,051,941 is only suitable for print heads where the electromagnetic structures are distributed circumferentially in a uniform way along a supporting member. In fact, the constructive arrangement provides for a disk shaped armature retainer, which is unitary for the whole set of armatures, and provides both biasing of the armatures at its periphery as well as the rest surface for the armatures which defines the air gap width. Such armature retainer is fixed to the electromagnetic structure support by means of a screw, central to it. Clearly such screw provides an imperfect constraint and the forces exerted on a sector of the armature retainer give a resulting force which is eccentric to the central screw causing a rotation (not negigible even if small) of the armature retainer at the constraint, with consequent slack at the retaining position and change of the air gap width.